Microscopic evidence shows that teeth grow in cyclic patterns, laying down marks of development and change much like the rings of a tree. Look closely at a tooth and you can see these tiny lines, which repeat over specific spans of time. You can also see disruptions in these patterns caused by periods of stress, including birth, disease, famine, and in some cases, death.

Because they are so durable, teeth are the most common fossils found. To a practiced eye, a single tooth is generally enough to determine the species of its former owner. Distinctive features include roots and cusps (protrusions in the chewing surface), which vary in number, shape, and size.

Chimp tooth, external view and cut in half, magnification ~5x

These wavy bands are Retzius lines, named for the 19th-century Swedish anatomist who first described them. In human teeth, Retzius lines form every 6 to 10 days. The exact timing is always the same for a given individual, though it varies between individuals and species. Why do our teeth have a week-long growth cycle? No one knows.

Retzius lines in a chimp tooth, magnification ~100x

These thin lines are known as cross-striations. They’re also called daily lines because one forms every 24 hours. Biological cycles called circadian rhythms create these and other regular patterns in growing teeth.

Cross-striations in a chimp tooth, magnification ~400x

Birth isn’t just stressful for mothers, it’s stressful for newborns, too. The shock of birth leaves a distinctive line known as the neonatal line. The neonatal line of this orangutan is indicated by a white arrow. Your teeth have neonatal lines, too.

Neonatal line in an orangutan tooth, magnification ~400x

Severe stress, famine, or illness that lasts more than a day disrupts the growth of teeth, leaving a telltale dark line. In this sample from an unknown human individual, severe prolonged stress, disease, or famine left visible dents in the tooth surface. (The black crust on top is dental plaque.)

Stress patterns in a human tooth, magnification ~100x

The blue arrow marks the “death line” on the tooth of a wild chimpanzee. This was the last layer of tooth to grow before death cut growth short. The numbers marking other areas of the tooth represent the chimp’s age, in days. In humans, teeth start growing before birth and don’t finish until about the age of 15. The last to finish growing are the back-most molars. If an individual dies while his or her teeth are still growing, it’s a simple—if painstaking—matter of counting the daily lines since birth to determine the age at death.